Two sortables stacked using z-index - jquery-ui

Question

I have this setup. JSFiddle link.
$('.item').draggable({
helper: 'clone',
connectToSortable: '.first-sortable, .second-sortable',
});
$('.first-sortable').sortable({
connectWith: '.second-sortable',
receive: function () {
$('.logger1').addClass('animated');
setTimeout(function () {
$('.logger1').removeClass('animated');
}, 300);
}
});
$('.second-sortable').sortable({
connectWith: '.first-sortable',
receive: function () {
$('.logger2').addClass('animated');
setTimeout(function () {
$('.logger2').removeClass('animated');
}, 300);
}
});
.item {
width: 50px;
height: 50px;
background: red;
display: inline-block;
margin-right: 5px;
}
.container {
position: relative;
margin-bottom: 20px;
}
.first-sortable {
height: 100px;
background: blue;
}
.second-sortable {
position: absolute;
left: 50%;
width: 200px;
transform: translateX(-50%);
top: 0;
bottom: 0;
background: rgba(3, 3, 3, .8);
z-index: 10;
padding-left: 20px;
}
.logger2,
.logger1 {
transition: all .3s color;
}
.animated {
color: blue;
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<script src="https://code.jquery.com/ui/1.11.4/jquery-ui.js"></script>
<div class="container">
<div class="first-sortable"></div>
<div class="second-sortable"></div>
</div>
<div class="item"></div>
<div class="logger1">Blue Sortable Receive</div>
<div class="logger2">Dark Sortable Receive</div>
The main idea is that, as you can see, is that the dark sortable is above the first one, it is positioned using position: absolute and z-index. I have a little bit of opacity for the black one, just so I can look what happens with the first sortable.
Below the bottom red square we have two indicators that shows when receive event is fired on the specific sortable.
Please note the fact that when you drop an item onto the blue sortable, only indicator for this sortable is fired. When I drop a new item onto the black a receive event is fired on both sortables.
Is this behavior a desired one? I think, in this case just the dark one have to receive this event.
I wonder if this is not jQuery UI bug. Ticket.